This invention relates to improvements in piston-driven valves. Such valves units include a valve body equipped with inlet and outlet openings the communication between which may be interrupted by a valve member which is integral with a rod connected to a piston which is subjected to a valve control pressure. The piston is located in a head mounted on the valve body and including a spring system securing the resetting of the driving-piston. This well-known valve type generally comprises two main variants:
(a) "normally closed" valves, where the driving piston, when at rest, is subjected to the action of its reset spring to hold the valve member on a valve seat, so that the valve opens only when the control pressure acts;
(b) "normally open" valves, where the driving piston, when at rest, is subjected to the action of the reset spring system to hold the valve member away from the valve seat, so that the valve closes only when the control pressure acts.
On the other hand, the operator must be able to have at his disposal valves operable under a high control pressure, which requires resetting systems consisting of high power reset springs located in heads of large dimensions. The high pressure valves are dangerous to handle, especially during disassembling operations for maintenance, due to the very high strength of the driving-piston reset springs.
The operator also must have at his disposal a relatively large number of valves for the various applications contemplated, i.e., normally open valves, normally closed valves, low control pressure valves, high control pressure valves, very high control pressure valves, etc.